System and method of interrogating implanted passive resonant-circuit devices

Abstract

A system and method for interrogating implanted passive resonant circuits is described. The resonant frequency is detected by transmitting a train of short pulses and then listening between the pulses for output of the resonant circuit. With proper spacing between the pulses, a vastly improved signal-to-noise ratio is obtained.

I claim: 
     
       1. A system for monitoring the pressure within the cranium of a living being, said system comprising: a resonant frequency circuit that is implanted within the cranium;   a remotely-located transmitter and a remotely-located receiver coupled through alternation means that operates said transmitter and receiver in alternation such that when said transmitter is transmitting an interrogation signal to said resonant frequency circuit said remotely-located receiver is de-activated and when said receiver is listening to a response signal from said resonant frequency circuit said transmitter is de-activated;   said interrogation signal comprising high frequency electromagnetic excitation waves wherein one of said excitation waves causes said resonant frequency circuit to resonate at an altered resonance frequency corresponding to the pressure of the cranium;   said remotely-located receiver detecting said altered resonance frequency in said response signal; and   a display, coupled to said remotely-located receiver, for displaying the pressure of the cranium corresponding to said detected altered resonance frequency.   
     
     
       2. The system of claim 1 wherein said alternation means comprises: a timing circuit;   a first switch coupled between said timing circuit and said remotely-located transmitter, said first switch having a first closed state that activates said transmitter and a first open state that de-activates said transmitter;   a second switch coupled between said timing circuit and said remotely-located receiver, said second switch having a second closed state that activates said receiver and a second open state for de-activating said receiver; and   said timing circuit closing said first switch in said first closed state while opening said second switch in said second open state and then opening said first switch in said first open state while closing said second switch in said second closed state.   
     
     
       3. The system of claim 2 wherein said first closed state is approximately 10 nanoseconds and said first open state is approximately 20 nanoseconds. 
     
     
       4. The system of claim 2 wherein said second closed state is approximately 10 nanoseconds and said second open state is approximately 20 nanonseconds. 
     
     
       5. The system of claim 3 wherein said first switch is a monolithic microwave integrated circuit. 
     
     
       6. The system of claim 4 wherein said second switch is a monolithic microwave integrated circuit. 
     
     
       7. The system of claim 1 wherein said transmitter comprises a voltage-controlled oscillator (VCO) and wherein said system further comprises a voltage sweeper, said voltage sweeper controlling said VCO to generate said high frequency electromagnetic excitation waves. 
     
     
       8. The system of claim 1 wherein said high frequency electromagnetic excitation waves are in the range of 3.8-3.82 GHz. 
     
     
       9. The system of claim 7 wherein said high frequency electromagnetic excitation waves are in the range of 3.8-3.82 GHz. 
     
     
       10. The system of claim 7 wherein said receiver comprises a peak detector for detecting said altered resonant frequency in said response signal. 
     
     
       11. The system of claim 10 wherein said peak detector comprises a half-wave rectifier, a direct current (DC) amplifier and a differentiator coupled together in series. 
     
     
       12. The system of claim 11 wherein said voltage sweeper generates a ramp voltage for controlling said VCO, said ramp voltage comprising linearly increasing portions and substantially vertical re-trace portions. 
     
     
       13. The system of claim 13 wherein said receiver further comprises logic means having a first input coupled to the output of said differentiator and a second input coupled to said voltage sweeper, said logic means having an output coupled to said display and wherein said logic means decouples said receiver from said display during said re-trace portions of said ramp voltage while permitting said display to display the pressure of the cranium when said altered resonance frequency is detected by said receiver. 
     
     
       14. A method for monitoring the pressure within the cranium of a living being, said method comprising the steps of: implanting a resonant frequency circuit within the cranium;   transmitting high frequency electromagnetic excitation waves from a transmitter to said resonant frequency circuit while precluding a receiver from receiving any response signal from said resonant frequency circuit during said transmitting and wherein one of said excitation waves causes said resonant frequency circuit to resonate at an altered resonance frequency corresponding to the pressure of the cranium;   precluding said transmitter from transmitting said high frequency electromagnetic waves while said receiver receives a response signal from said resonant frequency circuit;   detecting said altered resonance frequency in said response signal by said receiver; and   displaying the pressure of the cranium corresponding to said detected altered resonance frequency.   
     
     
       15. The method of claim 14 wherein said step of transmitting high frequency electromagnetic excitation waves from a transmitter to said resonant frequency circuit while precluding a receiver from receiving any response signal comprises de-activating said receiver for approximately 20 nanoseconds while activating said transmitter for approximately 10 nanoseconds. 
     
     
       16. The method of claim 15 wherein said step of precluding said transmitter from transmitting said high frequency electromagnetic waves while said receiver receives a response signal comprises de-activating said transmitter for approximately 20 nanoseconds while activating said receiver for approximately 10 nanoseconds. 
     
     
       17. The method of claim 14 wherein said step of transmitting high frequency electromagnetic excitation waves comprises sweeping said transmitter through the frequency range of 3.8-3.82 Ghz. 
     
     
       18. The method of claim 17 wherein step of sweeping said transmitter comprises using a ramp voltage control signal to sweep said transmitter, said ramp voltage control signal comprising linearly-increasing portions and substantially-vertical re-trace portions. 
     
     
       19. The method of claim 14 wherein said step of detecting said altered resonance frequency in said response signal comprises detecting the peak value in said response signal. 
     
     
       20. The method of claim 18 further comprising the step of decoupling said receiver from said display during said re-trace portions of said ramp voltage control signal.